1. Field of the Invention
The invention relates to a method and apparatus of controlling a hybrid power system. Specifically, this invention manages energy and power flow among one or more power generating sources, storage devices, loads, and the utility grid or an off grid power system, each of which is coupled to a common DC bus.
2. Discussion of the Related Art
In recent years, increased demands for energy and increased concerns about supplies of fossil fuels and their corresponding pollution have led to an increased interest in renewable energy sources. Two of the most common and best developed renewable energy sources are photovoltaic energy and wind energy. Other renewable energy sources may include fuel cells, hydroelectric energy, tidal energy, and biofuel or biomass generators. However, using renewable energy sources to generate electrical energy presents a new set of challenges.
One of the biggest challenges for connecting renewable energy sources to existing AC grids, whether the utility grid or an off-grid system, is that renewable energy sources provide a variable supply of energy. The supply may vary, for example, according to the amount of wind, cloud cover, or time of day. Further, different energy sources provide different types of electrical energy. A wind turbine, for example, is better suited to provide Alternating Current (AC) energy while a photovoltaic cell is better suited to provide Direct Current (DC) energy. As a result, combining multiple sources of renewable energy with other generating systems, such as the utility grid or independent micro turbines and generators, into a single system with an AC output requires integrating each of these different energy sources. Due to the variable nature of the energy supplied by renewable sources, it may be desirable to integrate a storage device in the power system. The energy storage device may be charged during periods of peak production by the renewable source or, alternately, by the utility grid or other generating source. The energy storage device may then supply energy back to the grid when the renewable source is generating less energy than is required by the loads in a grid-independent system or less than is commanded by the utility in a grid-tied system.
Prior attempts to integrate multiple sources of renewable energy typically require individually connecting each energy source as well as the storage device to a grid, where the grid may be either a stand-alone grid or the utility grid. Each manufacturer of a generating source, for example the wind turbine or the photovoltaic array, or load provides a power converter to connect the source or load to the grid. This approach typically results in an undesirable double power conversion, first converting the generating source to an AC voltage compatible with the utility grid and then back to a DC voltage compatible with the storage device.
Other attempts to integrate multiple sources have utilized a high-level controller managing energy flow by each of the devices. For example, multiple AC sources operating in parallel in a grid-independent system typically require transfer switches and a control scheme to select desired generating sources. Such integration systems typically require complex software customized for each system. Addition of other generating sources or loads to the system requires subsequent modification of the integration software and connection hardware. Consequently, this complex control scheme limits the flexibility of integrating future generating sources or other modifications into a power system.